Image sensor noise is comprised of fixed pattern noise (FPN) or temporal noise. FPN can be mitigated via offset and gain correction schemes. This, however, is not true for temporal noise. Once introduced, it remains in the image. Hence, temporal noise reduction techniques are required.
A major contributor of temporal noise in 3T pixels is reset noise with a noise power on the order of kT/C, where C is the capacitance at the sense node of the pixel. Reset noise results from random fluctuations in voltage at the sense node due to intrinsic noise in the reset transistor. These fluctuations are frozen on the sense node when the respective capacitance is disconnected from reset device. Reset noise can be suppressed by reducing the amount of these fluctuations. Below describes a method to reduce this component.
The noise reduction technique makes use of a column feedback amplifier and a tapered reset signal. The fluctuations on the sense node are reduced by sensing the variations instantaneously and adjusting the drain current by modulating gate of the reset transistor. The reset signal modulates the amplifier loop traversing hard reset and soft reset resulting in a noise power that is reduced by the open loop gain of the amplifier, i.e. kT/AC (where A is the amplifier gain), with low lag.
Moreover new techniques can be implemented if a photo-sensitive material, i.e. quantum film, is used instead of an integrated silicon photodiode, as the sense node is comprised of the electrical and silicon parasitic capacitance as well as the photosensitive material capacitance.